The present invention relates to a method for the electrochemical determination of the oxygen concentration with an oxygen sensor that comprises a working electrode.
The measurement of the partial oxygen pressure is an important analytical problem. A rapid and exact calculation of the oxygen value is necessary, particularly in medical technology. The determination of the oxygen in the blood of patients, for example, requires a precision of approximately 1 Torr, namely in the range between about 10 and 300 Torr. The drift of the signal should thereby not exceed a value of 5 Torr over the course of three days, this essentially corresponding to the duration of the measurement in the blood. Heretofore, oxygen sensors employed in medical technology, however, fall far short of satisfying these demands.
The oxygen sensor of Clark is currently still being generally utilized in medical technology (in this respect, see U.S. Pat. Nos. 2,913,386; 3,260,659 and 4,075,596). Although this sensor can, in fact, be employed in blood gas analyzers, it is not suitable for identifying the oxygen content in the blood, since an implantation or, respectively, a longer-lasting operation in the body is prevented by some critical sensor features. Included among them, for example, is a hydrophobic membrane that is arranged in front of the measuring electrode. The properties of this hydrophobic membrane are noticeably modified due to interactions in the body. Moreover, the constant and high oxygen consumption of the sensor leads to a pronounced rejection reaction that makes operation of the sensor even more difficult.
European Published Application 0 170 998 discloses a method for the electrochemical determination of the oxygen concentration, particularly in biological material, with an oxygen sensor that comprises a measuring electrode and a cooperating electrode. In this method, the measuring electrode has two potentials cyclically impressed on it and the current flowing during the measuring phase is interpreted as measured signal. The method mainly serves the purpose of measuring relative changes in the oxygen concentration in order to match the frequency of a heart pacemaker to the patient's requirements.
Given employment of oxygen sensors in intensive care, it is necessary to exactly identify the partial oxygen pressure, with optimally little drift, over a time span of several days. A method disclosed by German Published Application 40 14 109 is suitable for this purpose. In this method, the measuring or, respectively, sensor electrode (of the oxygen sensor) has a potential profile impressed on it having a plurality of potential steps, namely two measuring potentials and a quiescent potential. At a measuring potential, the current is thereby integrated in the potential range of the oxygen reduction; the other measuring potential lies above the reduction potential of oxygen, so that the current only covers the reactions occurring at the electrode. A signal for the partial oxygen pressure that is suitable for calibration is then obtained by forming the difference between the two integrals.
Measurements in vitro and in vivo in the blood have shown that the conventional method is not yet precise enough and low-drift enough for the required precision of only a few Torr deviation. Added thereto is that various electrodes have surfaces that differ in size, even given mass production, and, thus, have variable activity such that the measuring error can proceed beyond the required maximum error (in the oxygen pressure) of a few Torr.